CD137 (4-1BB, TNFRSF9) is a TNF receptor (TNFR) superfamily member and is expressed on activated CD4+ and CD8+ T cells, Treg, DC, monocytes, mast cells and eosinophils. CD137 activation plays an important role in CD8+ T cell activation and survival (Lee et al., 2002; Pulle et al., 2006). It sustains and augments, rather than initiates, effector functions and preferentially supports Th1 cytokine production (Shuford et al., 1997). In CD4+ T cells, CD137 stimulation initially results in activation and later in activation-induced cell death, explaining why CD137 agonistic antibodies have shown therapeutic effect in tumour immunity as well as in autoimmunity (Zhang, J C I, 2007, Sun, Trends Mol Med, 2003). CD137 also suppresses Treg function (So, Cytokine Growth Factor Rev, 2008). Activation of CD137 is dependent on receptor oligomerization (Rabu et al., 2005; Wyzgol et al., 2009).
CD137 agonistic antibody has been shown to activate endothelial cells in the tumour environment, leading to upregulation of ICAM-1 and VCAM-1 and improved T cell recruitment (Palazon, Cancer Res, 2011).
CD137 is upregulated on NK cells activated by cytokines or CD16, in mice or humans, respectively (see Melero, CCR 19 (5)1044-53, 2013 and references cited therein). CD137 has been shown to activate NK cells in mice as well as humans, potentiating ADCC (Kohrt et al., 2014), though there are reports suggesting opposite effects on NK cells in mice and humans, leading to NK cell activation in mice and inhibition in humans (Baessler, Blood, 2010).
Several studies have demonstrated induction of tumour immunity by treatment with agonistic CD137 antibody (Dubrot et al., 2010; Gauttier et al., 2014; Kim et al., 2001; McMillin et al., 2006; Melero et al., 1997; Miller et al., 2002; Sallin et al., 2014; Taraban et al., 2002; Uno et al., 2006; Vinay and Kwon, 2012; Wilcox et al., 2002). In addition, it synergizes with several immunomodulators, including CpG, TRAIL, CD40, OX-40, DRS, PD-1/PD-L1, CTLA-4 Tim-3, IL-2, IL-12 (Curran et al., 2011; Gray et al., 2008; Guo et al., 2013; Kwong et al., 2013; Lee et al., 2004; Morales-Kastresana et al., 2013; Pan et al., 2002; St Rose et al., 2013; Uno et al., 2006; Wei et al., 2013; Westwood et al., 2010; Westwood et al., 2014a; Westwood et al., 2014b) in pre-clinical models.
Two CD137 antibodies are in clinical development. Urelumab (BMS-66513) is a fully human IgG4 antibody developed by Bristol-Myers Squibb. Several phase I and II studies in various indications are currently ongoing. The other CD137 antibody in development is PF-05082566, a fully human IgG2 antibody developed by Pfizer. It is currently in phase I development in lymphoma and various solid cancers.
The agonistic effect of CD137 antibodies is affected by the isotype of the Fc region. The antibodies tested in the clinic are either IgG2 or IgG4. Like most TNFR family members, CD137 depends on cross linking for activation (Wilson 2011, Cancer Cell). The CD137L expressed on the membrane of an APC may induce significant multiple cross linking of the receptor. An antibody can by itself only cross link two CD137 receptors, and to induce a strong signal, further cross linking via FcγRs expressed on other cells (in trans) may be necessary for induction of a strong CD137 mediated signal. An exception to this may be IgG2 antibodies, which induce a cross linking independent signaling by an unknown mechanism (White et al, 2015 Cancer Cell). T cells do not express FcγRs, and the FcγR mediated cross linking in vivo is thought to be mediated by monocytes, macrophages, DCs and potentially B cells and other cell types. It has been suggested that interaction with the inhibitory FcγR FcγRIIB plays a major role for this effect in mouse models for CD40 agonists (Li 2011, Science), whereas for OX40 antibodies, interactions with activating receptors may be of greater importance (Bulliard 2014, Imm and Cell Biol). For CD137 antibodies, FcγRII is not critical (Sanmamed 2015, Semin Onc). The translational relevance of this is uncertain, since the human FcγR distribution as well as the affinity of different IgG isotypes to different FcγR differs from mice. Further, human IgG1 binds to mFcγRIIb with relatively low affinity, similar to mIgGIIa and considerably lower than mIgG1, the latter having the most potent effect in vivo (Li Science 2011, Overdijk 2012 JI, Horton et al 2008, White et al 2011 and 2014).
Another factor to take into account is that engagement of FcγR receptors may also induce ADCC, antibody-dependent cellular phagocytosis (ADCP) and complement-dependent cytotoxicity (CDC) on cells coated with antibodies (for simplicity ADCC below includes ADCP and CDC). Typically, human IgG1 is a strong inducer of NK/Macrophage dependent ADCC, depending on the nature of the target, the cell type and the receptor density. IgG4 antibodies may also induce ADCC but to a lower extent than IgG1 (Wang 2015, Front Imm; Vidarson 2014 Front Imm).
The effect of a CD137 agonistic antibody with different isotypes may thus be affected by the balance between 1) inducing cross linking, which results in a stronger immune activation, and 2) inducing ADCC, which may lead to killing of both effector T cells (predominantly CD8 T cells) and Tregs. The net effect of 1) and 2) will likely depend on the distribution of CD137 expressing cells, the possibility of the target cells to engage with FcγR expressing immune cells, the receptor density and affinity and the sensitivity of Teff vs Treg to ADCC. The CD137 expression is high both on CD8 and Tregs in melanoma tumours (Quezada, presentation SITC 2015). The IgG4 format would allow for FcγRI mediated cross linking by macrophages and monocytes, yet minimizing NK mediated ADCC of effector CD8 T cells.
However, as outlined above, it is difficult to translate comparison of different human Fc in mouse models due to differences in expression and affinity between murine and human FcRs. Further, the functional consequence in vivo of antibodies blocking the binding of the CD137L to CD137 is currently debated.
Several studies have demonstrated induction of tumour immunity by treatment with agonistic CD137 mAb (Dubrot et al., 2010; Gauttier et al., 2014; Kim et al., 2001; McMillin et al., 2006; Melero et al., 1997; Miller et al., 2002; Sallin et al., 2014; Taraban et al., 2002; Uno et al., 2006; Vinay and Kwon, 2012; Wilcox et al., 2002). Two different antibodies are commonly used for in vivo studies in mice, Lob12.3 and 3H3 (Shuford 1997 J Exp Med).
The toxicity seen in mouse models has been detected following repeated dosing in a time dependent but not dose dependent manner (Ascierto 2010 Semin Onc, Dubrot 2010 Can Imm, Niu 2007 JI). The toxicity includes skin toxicity and liver toxicity: aspartate amino transferase/alanine amino transferase ratio (ASAT/ALAT) and cytokine release. This suggests that either the toxicity requires CD137 mediated pre-activation of immune cell populations (likely T cells) or it depends on secondary effects caused by antidrug-antibodies (ADA) response, potentially forming aggregations of CD137 antibodies that may lead to enhanced cross-linking. The toxicities seen in mice are reversible and seems to depend on TNFa/CD8 cell dependent manner (Ascierto 2010 Sem Onc). Toxicology studies in monkeys showed that both single and repeated dosing of up to 100 mg/kg once weekly for four weeks was tolerable with no skin or liver toxicity detected (Ascierto 2010, Semin Onc).
Prolonged and continuous activation through TNF receptor family members may lead to immune exhaustion. Therefore, it may be of advantage to administer such antibodies in a manner allowing resting periods for the cells expressing the receptors. One approach to increase the resting period in a specific dosing protocol is to reduce the half-life of an antibody by for example decreasing the binding to the neonatal Fc receptor (FcRn). This could, depending on the administration route, also reduce the toxicity associated with the treatment.
There remains a need for improved anti-tumour therapies, particularly anti-CD137 antibodies suitable for clinical use and with improved properties, such as reduced toxicity.